Appropriate regulation of cellular metabolic programs involved in energy (ATP) generation is essential for normal tissue development and homeostasis. ATP generation in heart and slow-twitch skeletal muscle involves primarily oxidative metabolism of fatty acids and glucose within the mitochondrion. The capacity for mitochondria! ATP generation and the selection of energy substrates in heart and skeletal muscle is regulated by developmental, physiological, and environmental cues that signal coordinated changes in the levels of enzymes involved these processes. Perturbations of appropriate regulation lead to changes in mitochondrial metabolism and substrate selection linked to heart and skeletal muscle insulin resistance and diabetes, heart failure and age-related declines in muscle function. Nuclear receptor (NR) transcription factors and their coactivator, PGC-1a, play a central role in regulating energy metabolism in skeletal muscle, heart, brown adipose and liver, by coordinating changes in metabolic enzyme gene expression. The estrogen-related receptors (ERR), ERRa and ERRg, have been implicated as important metabolic regulators in heart and skeletal muscle, due to their high expression in these tissues and their responsiveness to physiologic cues that increase energy metabolism. In order to understand the broad regulatory program downstream of ERRs in muscle, we propose using in vitro cell models and genetically-modified mouse models of ERRa and ERRg gain- and loss-of-function. We will analyze these models with gene expression profiling coupled with chromatin immunoprecipitation to identify target genes for ERRs. Based upon gene targets identified to date, we propose that ERRs are essential activators of heart and skeletal muscle energy metabolism that play particularly important function in developmental switches in myocyte metabolic programs. We assess the regulation of metabolism using models in which ERR isoforms are selectively activated by overexpression or inhibited by gene targeting or small-interfering RNA. The characterization of ERR function in cardiac and skeletal muscle has important implications for disease states, including heart failure and myocardial ischemia, type 2 diabetes, and obesity. Our findings will elucidate the potential for ERRs as therapeutic targets for widespread human diseases associated with metabolic dysregulation.